1. 程式人生 > >Android Jetpack 之 Lifecycle

Android Jetpack 之 Lifecycle

前言

在日常的開發中,我們通常需要在 Activity / Fragment 的生命週期方法中進行一些繁重的操作,這樣使程式碼看起來十分臃腫。Lifecycle 的引入主要是用來管理和響應 Activity / Fragment 的生命週期的變化,幫助我們編寫出更易於組織且通常更加輕量級的程式碼,讓程式碼變得更易於維護。

Lifecycle 是一個類,它持有 Activity / Fragment 生命週期狀態的資訊,並允許其它物件觀察此狀態。

Lifecycle 使用

新增相關依賴

場景:讓 MVP 中的 Presenter 觀察 Activity 的 onCreate 和 onDestroy 狀態。

  • Presenter 繼承 LifecycleObserver 介面
interface IPresenter : LifecycleObserver {

    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    fun onCreate(owner: LifecycleOwner)

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    fun onDestroy(owner: LifecycleOwner)

    @OnLifecycleEvent(Lifecycle.Event.ON_ANY) // ON_ANY 註解能觀察到其它所有的生命週期方法
    fun onLifecycleChanged(owner: LifecycleOwner, event: Lifecycle.Event)
}
class MyPresenter : IPresenter {

    override fun onCreate(owner: LifecycleOwner) {
        Log.e(javaClass.simpleName, "onCreate")
    }

    override fun onDestroy(owner: LifecycleOwner) {
        Log.e(javaClass.simpleName, "onDestroy")
    }

    override fun onLifecycleChanged(owner: LifecycleOwner, event: Lifecycle.Event) {
//        Log.e(javaClass.simpleName, "onLifecycleChanged")
    }
}
  • 在 Activity 中新增 LifecycleObserver
class MyLifecycleActivity : AppCompatActivity() {

    private lateinit var myPresenter: MyPresenter

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_my_lifecycle)

        Log.e(javaClass.simpleName, "onCreate")

        myPresenter = MyPresenter()
        lifecycle.addObserver(myPresenter) // 新增 LifecycleObserver
    }

    override fun onDestroy() {
        Log.e(javaClass.simpleName, "onDestroy")
        super.onDestroy()
    }
}

啟動 Activity 會列印:

MyLifecycleActivity: onCreate
MyPresenter: onCreate

finish Activity 會列印:

MyPresenter: onDestroy
MyLifecycleActivity: onDestroy

以上 Presenter 物件只觀察了 Activity 的 onCreate 方法和 onDestroy 方法,我們還可以觀察其它的生命週期方法。在 Lifecycle 內部有個列舉類 Event , 它包含了 LifecycleObserver 能夠觀察到的所有生命週期方法,只需要新增上相應的註解即可。

enum class Event {
    /**
     * Constant for onCreate event of the [LifecycleOwner].
     */
    ON_CREATE,
    /**
     * Constant for onStart event of the [LifecycleOwner].
     */
    ON_START,
    /**
     * Constant for onResume event of the [LifecycleOwner].
     */
    ON_RESUME,
    /**
     * Constant for onPause event of the [LifecycleOwner].
     */
    ON_PAUSE,
    /**
     * Constant for onStop event of the [LifecycleOwner].
     */
    ON_STOP,
    /**
     * Constant for onDestroy event of the [LifecycleOwner].
     */
    ON_DESTROY,
    /**
     * An [Event] constant that can be used to match all events.
     */
    ON_ANY
}

Lifecycle 內部還有代表了各個生命週期所處狀態的列舉類 State

enum class State {

    /**
     * Destroyed state for a LifecycleOwner. After this event, this Lifecycle will not dispatch
     * any more events. For instance, for an [android.app.Activity], this state is reached
     * before Activity's [onDestroy] call.
     */
    DESTROYED,

    /**
     * Initialized state for a LifecycleOwner. For an [android.app.Activity], this is
     * the state when it is constructed but has not received
     * [onCreate] yet.
     */
    INITIALIZED,

    /**
     * Created state for a LifecycleOwner. For an [android.app.Activity], this state
     * is reached in two cases:
     *
     * after [onCreate] call;
     * before [onStop] call.
     */
    CREATED,

    /**
     * Started state for a LifecycleOwner. For an [android.app.Activity], this state
     * is reached in two cases:
     *
     * after [onStart] call;
     * before [onPause] call.
     */
    STARTED,

    /**
     * Resumed state for a LifecycleOwner. For an [android.app.Activity], this state
     * is reached after [onResume] is called.
     */
    RESUMED;

    /**
     * Compares if this State is greater or equal to the given `state`.
     *
     * @param state State to compare with
     * @return true if this State is greater or equal to the given `state`
     */
    fun isAtLeast(state: State): Boolean {
        return compareTo(state) >= 0
    }
}

在一般開發中,當 Activity 擁有多個 Presenter 並需要在各個生命週期做一些特殊邏輯時,程式碼可能是:

override fun onStop() {
    presenter1.onStop()
    presenter2.onStop()
    presenter3.onStop()
    super.onStop()
}

override fun onDestroy() {
    presenter1.onDestroy()
    presenter2.onDestroy()
    presenter3.onDestroy()
    super.onDestroy()
}

這樣會使 Activity 的程式碼變得很臃腫。

如果用 Lifecycle , 只需將持有 Lifecycle 物件的 Activity 的生命週期的響應分發到各個 LifecycleObserver 觀察者中即可。

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_my_lifecycle)
    
    lifecycle.addObserver(presenter1) // 新增 LifecycleObserver
    lifecycle.addObserver(presenter2) // 新增 LifecycleObserver
    lifecycle.addObserver(presenter3) // 新增 LifecycleObserver
}

基本原理

幾個概念

  • LifecycleObserver 介面

    Lifecycle 觀察者。實現了該介面的類,被 LifecycleOwner 類的 addObserver 方法註冊後,通過註解的方式即可觀察到 LifecycleOwner 的生命週期方法。

  • LifecycleOwner 介面

    Lifecycle 持有者。實現了該介面的類持有生命週期(Lifecycle 物件),該介面生命週期(Lifecycle 物件)的改變會被其註冊的觀察者 LifecycleObserver 觀察到並觸發其對應的事件。

  • Lifecycle 類

    生命週期。和 LifecycleOwner 不同,LifecycleOwner 通過 getLifecycle() 方法獲取到內部的 Lifecycle 物件。

  • State

    當前生命週期所處狀態。Lifecycle 將 Activity 的生命週期函式對應成 State .

  • Event

    當前生命週期改變對應的事件。State 變化將觸發 Event 事件,從而被已註冊的 LifecycleObserver 接收。

實現原理

LifecycleOwner

AppCompatActivity 的父類 SupportActivityFragment 一樣,實現了 LifecycleOwner 介面,因此它們都擁有 Lifecycle 物件。

public class SupportActivity extends Activity implements LifecycleOwner, Component {
   
    // ... 
   
    private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);

    public Lifecycle getLifecycle() {
        return this.mLifecycleRegistry;
    }
    
    // ...
}
public interface LifecycleOwner {
    /**
     * Returns the Lifecycle of the provider.
     *
     * @return The lifecycle of the provider.
     */
    @NonNull
    Lifecycle getLifecycle();
}

從原始碼可知 getLifecycle() 方法返回的是 LifecycleRegistry 物件,而 LifecycleRegistry 是 Lifecycle 的子類,所有對LifecycleObserver 的操作都是由 LifecycleRegistry 完成的。

LifecycleRegistry

生命週期登記處。作為 Lifecycle 的子類,它的作用是新增觀察者、響應生命週期事件和分發生命週期事件。

public class LifecycleRegistry extends Lifecycle {

    // LifecycleObserver Map , 每一個 Observer 都有一個 State
    private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap =
            new FastSafeIterableMap<>();

    // 當前的狀態
    private State mState;

    // Lifecycle 持有者,如繼承了 LifecycleOwner 的 SupportActivity
    private final WeakReference<LifecycleOwner> mLifecycleOwner;

    public LifecycleRegistry(@NonNull LifecycleOwner provider) {
        mLifecycleOwner = new WeakReference<>(provider);
        mState = INITIALIZED;
    }

   /**
    * 新增 LifecycleObserver 觀察者,並將之前的狀態分發給這個 Observer , 例如我們在 onResume 之後註冊這個 Observer , 
    * 該 Observer 依然能收到 ON_CREATE 事件
    */
    @Override
    public void addObserver(@NonNull LifecycleObserver observer) {
        // ...
        // 例如:Observer 初始狀態是 INITIALIZED , 當前狀態是 RESUMED , 需要將 INITIALIZED 到 RESUMED 之間的
        // 所有事件分發給 Observer
        while ((statefulObserver.mState.compareTo(targetState) < 0
                && mObserverMap.contains(observer))) {
            pushParentState(statefulObserver.mState);
            statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState));
            popParentState();
            // mState / subling may have been changed recalculate
            targetState = calculateTargetState(observer);
        }
        // ...
    }

    /**
     * 處理生命週期事件
     */
    public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
        State next = getStateAfter(event);
        moveToState(next);
    }

    /**
     * 改變狀態
     */
    private void moveToState(State next) {
        if (mState == next) {
            return;
        }
        mState = next;
        // ...
        sync();
        // ...
    }

    /**
     * 同步 Observer 狀態,並分發事件
     */
    private void sync() {
        LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
        if (lifecycleOwner == null) {
            Log.w(LOG_TAG, "LifecycleOwner is garbage collected, you shouldn't try dispatch "
                    + "new events from it.");
            return;
        }
        while (!isSynced()) {
            mNewEventOccurred = false;
            // State中,狀態值是從 DESTROYED - INITIALIZED - CREATED - STARTED - RESUMED 增大
            // 如果當前狀態值 < Observer 狀態值,需要通知 Observer 減小狀態值,直到等於當前狀態值
            if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
                backwardPass(lifecycleOwner);
            }
            Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
            // 如果當前狀態值 > Observer 狀態值,需要通知 Observer 增大狀態值,直到等於當前狀態值
            if (!mNewEventOccurred && newest != null
                    && mState.compareTo(newest.getValue().mState) > 0) {
                forwardPass(lifecycleOwner);
            }
        }
        mNewEventOccurred = false;
    }

    /**
     * 向前傳遞事件。
     * 增加 Observer 的狀態值,直到狀態值等於當前狀態值
     */
    private void forwardPass(LifecycleOwner lifecycleOwner) {
        Iterator<Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
                mObserverMap.iteratorWithAdditions();
        while (ascendingIterator.hasNext() && !mNewEventOccurred) {
            Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
            ObserverWithState observer = entry.getValue();
            while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
                    && mObserverMap.contains(entry.getKey()))) {
                pushParentState(observer.mState);
                // 分發狀態改變事件
                observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
                popParentState();
            }
        }
    }

    /**
     * 向後傳遞事件。
     * 減小 Observer 的狀態值,直到狀態值等於當前狀態值
     */
    private void backwardPass(LifecycleOwner lifecycleOwner) {
        Iterator<Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
                mObserverMap.descendingIterator();
        while (descendingIterator.hasNext() && !mNewEventOccurred) {
            Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();
            ObserverWithState observer = entry.getValue();
            while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
                    && mObserverMap.contains(entry.getKey()))) {
                Event event = downEvent(observer.mState);
                pushParentState(getStateAfter(event));
                observer.dispatchEvent(lifecycleOwner, event);
                popParentState();
            }
        }
    }
}

根據上面的分析,我們知道 LifecycleRegistry 才是真正替 Lifecycle 去埋頭幹粗活的類!

接下來繼續來看看實現了 LifecycleOwner 介面的 SupportActivity 類是如何將事件分發給 LifecycleRegistry 的。

public class SupportActivity extends Activity implements LifecycleOwner, Component {

    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ReportFragment.injectIfNeededIn(this);
    }
}

注意到 SupportActivity 的 onCreate() 方法裡面有行 ReportFragment.injectIfNeededIn(this) 程式碼,再進入 ReportFragment 類分析。

ReportFragment

public class ReportFragment extends Fragment {

    public static void injectIfNeededIn(Activity activity) {
        android.app.FragmentManager manager = activity.getFragmentManager();
        if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
            manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
            manager.executePendingTransactions();
        }
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        dispatchCreate(mProcessListener);
        dispatch(Lifecycle.Event.ON_CREATE);
    }

    @Override
    public void onStart() {
        super.onStart();
        dispatchStart(mProcessListener);
        dispatch(Lifecycle.Event.ON_START);
    }

    @Override
    public void onResume() {
        super.onResume();
        dispatchResume(mProcessListener);
        dispatch(Lifecycle.Event.ON_RESUME);
    }

    @Override
    public void onPause() {
        super.onPause();
        dispatch(Lifecycle.Event.ON_PAUSE);
    }

    @Override
    public void onStop() {
        super.onStop();
        dispatch(Lifecycle.Event.ON_STOP);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        dispatch(Lifecycle.Event.ON_DESTROY);
        // just want to be sure that we won't leak reference to an activity
        mProcessListener = null;
    }

    /**
     * 分發事件
     */
    private void dispatch(Lifecycle.Event event) {
        Activity activity = getActivity();
        if (activity instanceof LifecycleRegistryOwner) {
            ((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
            return;
        }

        if (activity instanceof LifecycleOwner) {
            Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
            if (lifecycle instanceof LifecycleRegistry) {
                ((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
            }
        }
    }
}

不難看出這是一個沒有 UI 的後臺 Fragment , 一般可以為 Activity 提供一些後臺行為。在 ReportFragment 的各個生命週期中都呼叫了 LifecycleRegistry.handleLifecycleEvent() 方法來分發生命週期事件。

為什麼不直接在 SupportActivity 的生命週期函式中給 Lifecycle 分發生命週期事件,而是要加一個 Fragment 呢?

在 ReportFragment 的 injectIfNeededIn() 方法中找到答案:

public static void injectIfNeededIn(Activity activity) {
    // ProcessLifecycleOwner should always correctly work and some activities may not extend
    // FragmentActivity from support lib, so we use framework fragments for activities
    android.app.FragmentManager manager = activity.getFragmentManager();
    if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
        manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
        // Hopefully, we are the first to make a transaction.
        manager.executePendingTransactions();
    }
}

有兩個原因:為了能讓 ProcessLifecycleOwner 正確地工作;②、並非所有的 Activity 都是繼承來自 support 包的 FragmentActivity 類的。因此封裝一個同樣具有生命週期的後臺 Fragment 來給 Lifecycle 分發生命週期事件。

另一方面,假如我們不繼承自 SupportActivity , 那 Lifecycle 是如何通過 ReportFragment 分發生命週期事件呢?

滑鼠停在 ReportFragment 類,同時按下 Ctrl + Shift + Alt + F7 在 Project and Libraries 的範圍下搜尋 ReportFragment 被引用的地方。我們發現還有 LifecycleDispatcher 和 ProcessLifecycleOwner 兩個類有使用到 ReportFragment .

LifecycleDispatcher

生命週期分發者。

class LifecycleDispatcher {

    // ...

    static void init(Context context) {
        if (sInitialized.getAndSet(true)) {
            return;
        }
        ((Application) context.getApplicationContext())
                .registerActivityLifecycleCallbacks(new DispatcherActivityCallback());
    }

    // 通過註冊 Application.registerActivityLifecycleCallbacks 來獲取 Activity 的生命週期回撥
    static class DispatcherActivityCallback extends EmptyActivityLifecycleCallbacks {
        private final FragmentCallback mFragmentCallback;

        DispatcherActivityCallback() {
            mFragmentCallback = new FragmentCallback();
        }

        @Override
        public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
            if (activity instanceof FragmentActivity) {
                ((FragmentActivity) activity).getSupportFragmentManager()
                        .registerFragmentLifecycleCallbacks(mFragmentCallback, true);
            }
            // 給每個 Activity 新增 ReportFragment
            ReportFragment.injectIfNeededIn(activity);
        }

        @Override
        public void onActivityStopped(Activity activity) {
            if (activity instanceof FragmentActivity) {
                markState((FragmentActivity) activity, CREATED);
            }
        }

        @Override
        public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
            if (activity instanceof FragmentActivity) {
                markState((FragmentActivity) activity, CREATED);
            }
        }
    }

    /**
     * 通過遞迴形式給所有子 Fragment 設定 State
     */
    private static void markState(FragmentManager manager, State state) {
        Collection<Fragment> fragments = manager.getFragments();
        if (fragments == null) {
            return;
        }
        for (Fragment fragment : fragments) {
            if (fragment == null) {
                continue;
            }
            markStateIn(fragment, state);
            if (fragment.isAdded()) {
                // 遞迴
                markState(fragment.getChildFragmentManager(), state);
            }
        }
    }

    private static void markStateIn(Object object, State state) {
        if (object instanceof LifecycleRegistryOwner) {
            LifecycleRegistry registry = ((LifecycleRegistryOwner) object).getLifecycle();
            registry.markState(state);
        }
    }

    /**
     * 將某 Activity 及其所有子 Fragment 的 State 設定為某狀態 
     */
    private static void markState(FragmentActivity activity, State state) {
        markStateIn(activity, state);
        markState(activity.getSupportFragmentManager(), state);
    }

    // ...
}

從原始碼可知,LifecycleDispatcher 是通過註冊 Application.registerActivityLifecycleCallbacks 來監聽 Activity 的生命週期回撥的。

  • 在 onActivityCreated 中新增 ReportFragment , 將 Activity 的生命週期交給 ReportFragment 去分發給 LifecycleRegistry ;
  • 在 onActivityStopped() 以及 onActivitySaveInstanceState() 中,將 Activity 及其所有子 Fragment 的 State 置為 CREATED .

ProcessLifecycleOwner

為整個 App 程序提供生命週期的類。

public class ProcessLifecycleOwner implements LifecycleOwner {

    static final long TIMEOUT_MS = 700; //mls

    // ...

    static void init(Context context) {
        sInstance.attach(context);
    }

    private ActivityInitializationListener mInitializationListener =
            new ActivityInitializationListener() {
                @Override
                public void onCreate() {
                }

                @Override
                public void onStart() {
                    activityStarted();
                }

                @Override
                public void onResume() {
                    activityResumed();
                }
            };

    void activityStarted() {
        mStartedCounter++;
        if (mStartedCounter == 1 && mStopSent) {
            mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START);
            mStopSent = false;
        }
    }

    void activityResumed() {
        mResumedCounter++;
        if (mResumedCounter == 1) {
            if (mPauseSent) {
                mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
                mPauseSent = false;
            } else {
                mHandler.removeCallbacks(mDelayedPauseRunnable);
            }
        }
    }

    void activityPaused() {
        mResumedCounter--;
        if (mResumedCounter == 0) {
            mHandler.postDelayed(mDelayedPauseRunnable, TIMEOUT_MS);
        }
    }

    void activityStopped() {
        mStartedCounter--;
        dispatchStopIfNeeded();
    }

    void attach(Context context) {
        mHandler = new Handler();
        mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
        Application app = (Application) context.getApplicationContext();
        app.registerActivityLifecycleCallbacks(new EmptyActivityLifecycleCallbacks() {
            @Override
            public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
                ReportFragment.get(activity).setProcessListener(mInitializationListener);
            }

            @Override
            public void onActivityPaused(Activity activity) {
                activityPaused();
            }

            @Override
            public void onActivityStopped(Activity activity) {
                activityStopped();
            }
        });
    }
}

從原始碼可知:

  • ProcessLifecycleOwner 是用來監聽 Application 生命週期的,它只會分發一次 ON_CREATE 事件,並不會分發 ON_DESTROY 事件;
  • ProcessLifecycleOwner 在 Activity 的 onResume 中呼叫 Handle.postDelayed() , 在 onPause 中呼叫了 mHandler.removeCallbacks(mDelayedPauseRunnable) , 是為了處理 Activity 重建時比如橫豎螢幕切換時,不會發送事件;
  • ProcessLifecycleOwner 一般用來判斷應用是在前臺還是後臺,但由於使用了 Handle.postDelayed() , TIMEOUT_MS = 700,因此這個判斷不是即時的,有 700ms 的延遲;
  • ProcessLifecycleOwner 與 LifecycleDispatcher 一樣,都是通過註冊 Application.registerActivityLifecycleCallbacks 來監聽 Activity 的生命週期回撥,來給每個 Activity 新增 ReportFragment 的。

最後,通過點選 init() 方法,我們發現 LifecycleDispatcher 和 ProcessLifecycleOwner 都是在 ProcessLifecycleOwnerInitializer 類下完成初始化的,而 ProcessLifecycleOwnerInitializer 是一個 ContentProvider .

public class ProcessLifecycleOwnerInitializer extends ContentProvider {
    
    @Override
    public boolean onCreate() {
        LifecycleDispatcher.init(getContext());
        ProcessLifecycleOwner.init(getContext());
        return true;
    }

    // ...
}

Lifecycle 會自動在我們的 AndroidManifest.xml 中新增以下程式碼用於初始化 ProcessLifecycleOwner 與 LifecycleDispatcher , 這樣就不需要我們在 Application 中寫程式碼來初始化了。

<manifest xmlns:android="http://schemas.android.com/apk/res/android">
  // ...
  <provider
    android:name="android.arch.lifecycle.ProcessLifecycleOwnerInitializer"
    android:authorities="me.baron.achitecturelearning.lifecycle-trojan"
    android:exported="false"
    android:multiprocess="true" />
</manifest>

Lifecycle 的最佳實踐

  • 保持 Activity / Fragment 儘可能的精簡,它們不應該試圖去獲取它們所需的資料,要用 ViewModel 來獲取,並觀察 LiveData 物件將資料變化反映到檢視中;
  • 嘗試編寫資料驅動(data-driven)的 UI , 即 UI 控制器的責任是在資料改變時更新檢視或者將使用者的操作通知給 ViewModel ;
  • 將資料邏輯放到 ViewModel 類中,ViewModel 應該作為 UI 控制器和應用程式其它部分的連線服務。注意:不是由 ViewModel 負責獲取資料(例如:從網路獲取)。相反,ViewModel 呼叫相應的元件獲取資料,然後將資料獲取結果提供給 UI 控制器;
  • 使用 Data Binding 來保持檢視和 UI 控制器之間的接口乾淨。這樣可以讓檢視更具宣告性,並且儘可能減少在 Activity 和 Fragment 中編寫更新程式碼。如果你喜歡在 Java 中執行該操作,請使用像 Butter Knife 這樣的庫來避免使用樣板程式碼並進行更好的抽象化;
  • 如果 UI 很複雜,可以考慮建立一個 Presenter 類來處理 UI 的修改。雖然通常這樣做不是必要的,但可能會讓 UI 更容易測試;
  • 不要在 ViewModel 中引用 View 或者 Activity 的 context . 因為如果 ViewModel 存活的比 Activity 時間長(在配置更改的情況下),Activity 將會被洩漏並且無法被正確的回收。

文中 Demo GitHub 地址

參考資料: